1. Field of Invention
This invention relates to the field of printing, particularly to the method for making relief deepenings in hydrophylic, adhesive printing plate materials by using a contact-separation motion to create and terminate an adhesive bond between the plate surface made adhesive and the removal medium.
2. Description of Prior Arts
Technological advances in the art of printing introduced new processes with emphasis on fast production of good quality printing plates for direct use on rotary printing presses, especially in the newspaper industry. Metal plates were found not to fulfill this requirement because they are slow in being processed, emit health hazardous fumes and are not ready for direct use.
Printing plates, particularly photopolymer water soluble plates, alleviated most of these problems. However, present processing methods are only satisfactory and require a negative. These methods utilizing pressurized spraying of the plate surface with heated water are primitive, not refined. They do not exert any control on the shaping and forming of sidewalls supporting the image. The formation of said sidewalls is determined by the hardening action of light which the plate receives at the time of exposure. Aforesaid demands a sharp, uniformly dense negative and perfect setting and functioning of the light source, factors which need due care and are not always achievable. It is known that the depth of relief is limited to the depth hardening effect of the light. Material not sufficiently hardened is washed away during processing. The relationship between the light hardening action and the reproductive quality of the plate is therefore established. Thus extended variations in the opacity of the paste-up or any deviations in the quantum of light endanger the reproductive quality of the plate.
Prior art techniques are also known to be inefficient in processing plates with halftone lineage higher than 65 lines, nor do said techniques process fine lines in reverse areas to the sufficient depth for quality reproduction.
Another impediment of these techniques is that the temperature of the plate rises to the temperature of the spraying solution and is kept at this high temperature during the entire processing time thereby rendering the plate vulnerable to fine detail washout. Due to the length of processing time said temperatures also cause partial hardening of plate material, hindering the removal efficiency of these methods resulting in shallowness of depth in reverse areas thereby reducing sharpness of image contrast in said areas of image. According to the manufacturer's instructions temperatures as high as 130 to 140 degrees may be encountered.
Because said techniques need an excessive amount of water to dissolve and wash away unhardened plastic material, the biodegradable breakdown process only reduces water pollution, it does not eliminate it. Furthermore while these techniques have shortened washout time to 3 to 4 minutes this amount is still considered lengthy for newspaper production where speed is so essential.
U.S. Pat. No. 3,661,660 to Wessells et al. and Patent teachings of Kleeberg et al. U.S. Pat. No. 3,957,512 and Takeda et al. U.S. Pat. No. 4,050,936 are limited to certain light sensitive chemical substances requiring action of light to cause explicit chemical changes in plate material. Reliability of said changes is entirely dependent on uniformity of light source and opacity of a negative thus being much more vulnerable to deviate than a non-chemical removal action as taught in present application. Furthermore said patents neither teach how to make relief in plate material when the image is formed without use of light nor how to control and form image supporting sidewalls as illustrated in present application FIG. 2; sidewalls designed to redress paper abrasive effect on image fine meshed screen parts.
Etching methods as taught in U.S. Pat. No. 3,271,226 to H. C. Steahle et al. and U.S. Pat. No. 3,411,973 to H. K. Siler are applied to hard plastic material softened by chemical solutions other than water and rely exclusively on a brushing motion to etch the plate. Said methods do not complete the etching step in one uninterrupted pass. The supporting treatment must be incorporated in the process, rendering these techniques time consuming. The structural characteristics of hydrophylic plates are much more responsive to absorbency and diffusion of water than cellulosic plate materials to semisolvents when treated as taught by Siler. Thus it is almost impossible to keep the plate surface under the stencilled image dry; a critical and imperative prerequisite of Siler's teaching. Further impediments of Siler's teaching when applied to hydrophylic printing plates are horizontal, uncontrolled movement of brush bristles and limitation of the amount of applied pressure on said bristles so as not to change brushing action to ineffective wiping.
Adaptability of Misek's teaching U.S. Pat. No. 3,874,945 to hydrophylic adhesive materials is difficult and limited. Basic principle of said Patent is to selectively apply solution to the plate by an etching medium and remove dissolved plate material by remaining absorbency of said medium or by blotting paper. When said method is applied to hydrophylic plate material the absorbency and diffusion of this material, when treated with water, is so great that said treatment impairs selectivity of etching medium application. Also capacity of etching medium to hold enough solution to dissolve hydrophylic plate material on contact is not feasible. When other means such as a brush is used to supply solution to the plate the ability to form and control image supporting sidewalls of said teaching is lost. Furthermore removal of plate dissolved material when in the form of liquid demands many more removal applications than when said material is only partially liquefied and is removed by adhesion. The present application teaches the removal of plate material by adhesion. To redress aforementioned impediments of Misek's Patent the present method divides the treatment of plate surface and removal of plate material into two separate functions independent of each other. Thus the treatment of plate surface with water can be controlled by omitting or repeating of this step in the processing cycle and action of removal medium can be enhanced by the supporting layer of resilient material so image sidewalls designated for long run as illustrated in FIG. 2 of present application are attainable. It was also found that supporting the etching medium by small rollers as taught in Misek's patent is ineffective in halftone areas of the image.
Thus previously described procedures are not feasible nor readily adaptable for use on water soluble, adhesive plate materials. They do not exploit advantages of said plates pertaining to speed, quality and ease of processing. Nor can they be adapted to said materials when image on the plate surface is formed by direct imaging without the use of light and a negative and said image acts as a stencil during plate processing.